®

LOG100

7

The frequency response curves are shown for constant DC I

1

and I

The transient response of the LOG100 is different for in-

creasing and decreasing signals. This is due to the fact that

a log amp is a nonlinear gain element and has different gains

at different levels of input signals. Frequency response

decreases as the gain increases.

GENERAL INFORMATION

INPUT CURRENT RANGE

The stated input range of 1nA to 1mA is the range for

specified accuracy. Smaller or larger input currents may be

applied with decreased accuracy. Currents larger than 1mA

result in increased nonlinearity. The 10mA absolute maxi-

mum is a conservative value to limit the power dissipation

in the output stage of A

below 1nA will result in increased errors due to the input

bias currents of A

be nulled. See Optional Adjustments section.

FREQUENCY COMPENSATION

Frequency compensation for the LOG100 is obtained by

connecting a capacitor between pins 7 and 14. The size of

the capacitor is a function of the input currents as shown in

the Typical Performance Curves. For any given application,

the smallest value of the capacitor which may be used is

determined by the maximum value at I

value of I

stable, but will reduce the frequency response.

SETTING THE REFERENCE CURRENT

When the LOG100 is used as a straight log amplifier I

constant and becomes the reference current in the expression

V

(21)

I

shown in Figure 4), or it may be derived from a voltage

source with one or more resistors.

When a single resistor is used, the value may be quite large

when I

R

= 1500M

Ω.

FIGURE 5. “T” Network for Reference Current.

A

1

+

R

2

V

REF

R

1

R

3

V

T

I

REF

–

V

OS

14

A voltage divider may be used to reduce the value of the

resistor. When this is done, one must be aware of possible

errors caused by the amplifier’s input offset voltage. This is

shown in Figure 5.

In this case the voltage at pin 14 is not exactly zero, but is

equal to the value of the input offset voltage of A

ranges from zero to

±5mV. V

than 5mV in order to make this effect negligible. This

concept also applies to pin 1.

OPTIONAL ADJUSTMENTS

The LOG100 will meet its specified accuracy with no user

adjustments. If improved performance is desired, the follow-

ing optional adjustments may be made.

INPUT BIAS CURRENT

The circuit in Figure 6 may be used to compensate for the

input bias currents of A

FET inputs with the characteristic bias current doubling

every 10

°C, this nulling technique is practical only where

the temperature is fairly stable.

FIGURE 4. Temperature-Compensated Current Reference.

I

REF

I

1

15V

10nA

2N2905

I

REF

R

REF

2N2905

+15V

–15V

I

6V

R

REF

3.6k

Ω

6V

IN834

FIGURE 6. Bias Current Nulling.

–V

CC

R

1kM

Ω

I

2

I

1

R

10k

Ω

R

1

1kM

Ω

R

2

10k

Ω

+V

CC

14

1

5

4

3

6

10

7

V

OUT

9

–V

CC

+V

CC

C

C

+

–

LOG100

OUTPUT OFFSET

The output offset may be nulled with the circuit in Figure 7.

I

of 100nA to 100

µA. R

voltage.